The present invention relates to the field of simulating the propagation of radiofrequency signals. More specifically, the present invention relates to identifying multipath components of a radiofrequency signal within a three-dimensional environment.
In wireless communications systems, such as cellular, Personal Communications Services, and so forth, base stations are located such that radio signals are available throughout the service area. As a radiofrequency signal propagates from a transmitter, the signal can reach the receiving antenna by two or more paths (i.e., multipaths). This phenomena is especially evident in cluttered environments, such as in urban areas containing many tall buildings. The effects of multipaths are signal distortions due to signal interference at the receiver because of differences in arrival times and received power for waves with propagation paths of different lengths, differences in Doppler shift at a moving receiver or paths with different angles of arrival, and so forth. Predicting multipaths of a radiofrequency signal and their effect on radio signal quality is a daunting endeavor.
A number of tools have been developed which make use of terrain data, with building clutter information, satellite imagery, and so forth. This data is used in conjunction with models which use base and subscriber heights, along with a description of the terrain to predict multipath effects for the locations under consideration.
Such models may work adequately for large cells whose base antennas are well above the height of the terrain and the building clutter, so the influence of particular terrain, buildings, or groups of buildings is minimal. However, when the base station antennas are near rooftop level or below building rooftops, then the size and shape of the buildings significantly influence the radiofrequency signals as they propagate down the streets and reflect off of the buildings.
Ray tracing processes attempt to model the propagation of radiofrequency signals as rays radiating from the transmitter to the receiver. Within ray tracing there are two generally known approaches. The first is called the xe2x80x9cshooting-and-bouncingxe2x80x9d method, in which a fixed number of rays are launched from a transmitter, then forward-traced to follow the different propagation paths, with a ray being terminated when it hits a detection sphere at the receiver. A problem with this method is that the location of not only the transmitter but also the receiver must be known prior to forward-tracing the propagation paths. Thus, the rays have to be launched and traced again in all directions for each additional receiver location. This could mean hours or even days of computation time for a practical outdoor environment. Another problem arises with identifying which of the multipaths represent distinct wavefronts. In a radiofrequency signal, the wavefront is the surface that is defined by the locus of points that have the same phase, i.e., the same path length, from the source. If more than one propagation path represents the same wavefront, these propagation paths may be counted as contributions to the overall power at the receiver location, resulting in an inaccurately optimistic power level calculation for the receiver location.
The second approach is based on image theory. The basic assumption in image theory is that the images of a source at a fixed location in a given environment are independent of the location of the receiver as long as there are planar surfaces in the environment. Therefore one can build all the images for a given location of the transmitter and environment and reuse it for as many receiver locations as one needs. This represents an improvement in terms of computational efficiency; however, the image method becomes too cumbersome with large numbers of randomly oriented polygons in the environment.
A conventional image theory approach is to first determine an image tree (hierarchically organized for ease of use) based on the location of the transmitter in the environment and the environment itself. The environment consists of reflective surfaces and corners. Starting from the transmitter (xe2x80x9cparentxe2x80x9d image), each reflective surface or corner has the potential of generating a xe2x80x9cchildxe2x80x9d image from the parent image. Each child image can further generate child images for every reflective surface and every corner. Once the image tree is built, for a given receiver location, every image on the tree is examined to see whether it contributes to the total received power through a back-tracing process from the receiver to the transmitter. A problem with image tracing is the large size of the image tree for a realistic outdoor environment leads to very large computational and memory requirements.
In order to limit the size of the image tree, one prior art image tree method creates a parent image node associated with the transmitter. Child image nodes are created in the image tree only for each object (i.e., reflective surface or diffractive corner) that can redirect a propagating signal from the transmitter when the power received at the object exceeds a threshold. The power in the reflected signal depends on the distance traveled to the object, the incident angle with the panel, and the reflection coefficient of object. Accordingly, the size of the image tree created using this approach is significantly reduced from prior art image trees. Unfortunately, since power computations have to be performed for every object and every propagating signal, again the problem arises of prohibitively long computation time to create the image tree.
Accordingly, it is an advantage of the present invention that a method and system for characterizing propagation of a radiofrequency signal in a three-dimensional environment are provided.
It is another advantage of the present invention that the method and system allow a database of propagation paths to be created separate from the selection of multipaths at a receiver location.
It is another advantage of the present invention that the database can be used to select multipaths at any of a number of receiver locations.
It is another advantage of the present invention that the method and system readily select multipath components in the absence of power calculations.
Yet another advantage of the present invention is that the method and system identify distinct wavefronts at a receiver location.
The above and other advantages of the present invention are carried out in one form by a method for characterizing propagation of a radiofrequency signal in a three-dimensional environment. The method calls for forming a database of propagation paths of the radiofrequency signal transmitted from a transmitter at a fixed location in the three-dimensional environment, the propagation paths being represented by a plurality of line segments in the database. Local line segments are selected from the plurality of line segments to establish a subset of the propagation paths, the local line segments being proximate a location of a receiver in the three-dimensional environment, and each of the propagation paths of the subset including one of the local line segments. The method further calls for identifying distinct multipath components of the radiofrequency signal from the subset of the propagation paths.
The above and other advantages of the present invention are carried out in another form by a system for determining propagation characteristics of a radiofrequency signal between a transmitter location and each of a plurality of receiver locations in a three-dimensional environment. The system includes a memory element having a database of propagation paths stored therein. The propagation paths are represented by a plurality of line segments, and the propagation paths originate at the transmitter location and radially project from the transmitter location along an azimuth and an elevation such that adjacent ones of the propagation paths are spaced apart by an angle of separation. The system further includes a location coordinates receiver for one of the receiver locations, and a segment selector for selecting, in response to the location coordinates, local line segments from the plurality of line segments in the database to establish a subset of the propagation paths, the local line segments being proximate the one receiver location, and each of the propagation paths of the subset including one of the local line segments.